Carbon/carbon (“C/C”) parts are employed in various industries. An exemplary use for carbon/carbon parts is fabricating them in the form of friction disks such as aircraft brake disks, race car brake disks, and clutch disks. C/C brake disks are especially useful in such applications because of the superior high temperature characteristics of C/C material. In particular, the C/C material used in C/C parts is a good conductor of heat and the C/C material is able to dissipate heat away from the braking surfaces that is generated in response to braking. C/C material is also highly resistant to heat damage, and is thus capable of sustaining friction between brake surfaces during severe braking without a significant reduction in the friction coefficient or mechanical failure.
C/C material is generally formed by utilizing continuous oxidized polyacrylonitrile (PAN) fibers, referred to as “OPF” fibers. These OPF fibers are the precursors of carbonized PAN fibers and are used to fabricate a preformed shape using a needle punching process. OPF fibers are layered in a selected orientation into a preform of a selected geometry. Typically, two or more layers of fibers are layered onto a support and are then needled together simultaneously or in a series of needling steps. This process interconnects the horizontal fibers with a third direction also called the z-direction, and the fibers extending into the third direction are also called z-fibers. This needling process may involve driving a multitude of barbed needles into the fibrous layers to displace a portion of the horizontal fibers into the z-direction. Transport of the fibers by the barbed needles is a function of several parameters including barb geometry, barb position with respect to the orientation of the fibers, needle penetration depth and fiber characteristics. Fibers presenting a high modulus like carbon fibers may be difficult to transport in the z-direction as they may break under the action of the needles. The polymeric nature of the OPF fibers facilitates the transport of the low modulus fibers in the z direction. Controlling the degree of transport in the z-direction and the distribution of z-fibers may be important steps in the production of a preform.
After the needling process is complete, the OPF fibers are generally carbonized at high temperatures in a controlled environment to transform the preform into a high carbon content substrate. Following this batch operation, the resulting fibrous carbon substrate is densified through chemical vapor deposition of pyrolytic carbon until the composite reaches the desired density level.
Alternatively, and to eliminate or reduce the costly and lengthy carbonization step, the fibrous preform may be prepared directly from carbonized carbon fibers or from stabilized carbon precursor such as stabilized pitch fibers. Carbonized carbon fibers are readily commercially available in substantially continuous tow forms from a wide number of suppliers and possess desirable higher thermal and mechanical properties than the transformed OPF fibers. Stabilized pitch fibers are also commercially available and have potential to further increase the final composite thermal properties. Additionally, other inorganic fibers such as those prepared with a ceramic precursor may be available as well. Such fibers, however, exhibit a high modulus and are much more difficult to transport in a substantially continuous tow form in the z-direction with barbed needles.
For example, large lengths of carbonized fibers, when constrained at both ends, will generally break under a downward movement of the barbed needles rather than being transported into the preform like PAN/OPF fibers. Alternatively, short lengths of pre-cut carbonized fibers may be added in the form of loose fibers or a web. Loose fibers are not amenable to a well controlled production process and a web of suitable geometry may not be readily commercially available. In addition, a preform prepared with a high number of short length fibers may exhibit low strength.
Therefore, a method for increasing the amount of displaced fibers into the z-direction from a starting, substantially continuous textile is desired.